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Re: [PATCH] hvmloader: pass PCI MMIO layout to OVMF as an info table

On 11/01/2021 09:27, Jan Beulich wrote:
> On 11.01.2021 05:53, Igor Druzhinin wrote:
>> We faced a problem with passing through a PCI device with 64GB BAR to
>> UEFI guest. The BAR is expectedly programmed into 64-bit PCI aperture at
>> 64G address which pushes physical address space to 37 bits. OVMF uses
>> address width early in PEI phase to make DXE identity pages covering
>> the whole addressable space so it needs to know the last address it needs
>> to cover but at the same time not overdo the mappings.
>> As there is seemingly no other way to pass or get this information in
>> OVMF at this early phase (ACPI is not yet available, PCI is not yet 
>> enumerated,
>> xenstore is not yet initialized) - extend the info structure with a new
>> table. Since the structure was initially created to be extendable -
>> the change is backward compatible.
> How does UEFI handle the same situation on baremetal? I'd guess it is
> in even more trouble there, as it couldn't even read addresses from
> BARs, but would first need to assign them (or at least calculate
> their intended positions).

Maybe Laszlo or Anthony could answer this question quickly while I'm 

>> --- a/tools/firmware/hvmloader/ovmf.c
>> +++ b/tools/firmware/hvmloader/ovmf.c
>> @@ -61,6 +61,14 @@ struct ovmf_info {
>>      uint32_t e820_nr;
>>  } __attribute__ ((packed));
>> +#define OVMF_INFO_PCI_TABLE 0
>> +struct ovmf_pci_info {
>> +    uint64_t low_start;
>> +    uint64_t low_end;
>> +    uint64_t hi_start;
>> +    uint64_t hi_end;
>> +} __attribute__ ((packed));
> Forming part of ABI, I believe this belongs in a public header,
> which consumers could at least in principle use verbatim if
> they wanted to.

It probably does, but if we'd want to move all of hand-over structures
wholesale that would include seabios as well. I'd stick with the current
approach to avoid code churn in various repos. Besides the structures
are not the only bits of ABI that are implicitly shared with BIOS images.

>> @@ -74,9 +82,21 @@ static void ovmf_setup_bios_info(void)
>>  static void ovmf_finish_bios_info(void)
>>  {
>>      struct ovmf_info *info = (void *)OVMF_INFO_PHYSICAL_ADDRESS;
>> +    struct ovmf_pci_info *pci_info;
>> +    uint64_t *tables = scratch_alloc(sizeof(uint64_t)*OVMF_INFO_MAX_TABLES, 
>> 0);
> I wasn't able to locate OVMF_INFO_MAX_TABLES in either
> xen/include/public/ or tools/firmware/. Where does it get
> defined?

I expect it to be unlimited from OVMF side. It just expects an array of 
tables_nr elements.

> Also (nit) missing blanks around * .
>>      uint32_t i;
>>      uint8_t checksum;
>> +    pci_info = scratch_alloc(sizeof(struct ovmf_pci_info), 0);
> Is "scratch" correct here and above? I guess intended usage /
> scope will want spelling out somewhere.

Again, scratch_alloc is used universally for handing over info between hvmloader
and BIOS images. Where would you want it to be spelled out?

>> +    pci_info->low_start = pci_mem_start;
>> +    pci_info->low_end = pci_mem_end;
>> +    pci_info->hi_start = pci_hi_mem_start;
>> +    pci_info->hi_end = pci_hi_mem_end;
>> +
>> +    tables[OVMF_INFO_PCI_TABLE] = (uint32_t)pci_info;
>> +    info->tables = (uint32_t)tables;
>> +    info->tables_nr = 1;
> In how far is this problem (and hence solution / workaround) OVMF
> specific? IOW don't we need a more generic approach here?

I believe it's very OVMF specific given only OVMF constructs identity page
tables for the whole address space - that's how it was designed. Seabios to
the best of my knowledge only has access to lower 4G.




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